This application claims the priority of Japanese Patent Applications No. 207487/1992 filed Jul. 10, 1992, which is incorporated herein by reference.
Japanese Patent Publication No. 57-29067 (29067/1982) proposed a structure of a light emitting device produced by the steps of boring a circular concavity on a front surface of a chip above an emanating region, supplying adhesive on the concavity and fixing a spherical lens on the concavity by the adhesive. The structure succeeded in the correct positioning of the lens, because the lens circularly contacts with the verge of the concavity. If the center of the emanating region coincides with the center of the concavity, the center of the lens coincides with the center of the emanating region. However, since painting of adhesive on the concavity precedes a spherical lens fitting on the adhesive-painted concavity, the adhesive with high viscosity is not excluded enough from the concavity. Then, the spherical lens is not able to come into contact with the verge of the concavity, floating on the adhesive away from the edge of the concavity. This is a difficulty of Japanese Patent Publication No. 57-29067.
Another drawback is the deviation of the center of the emanating region from the center of the concavity. The center of the emanating region does not always coincide with the center of the concavity in the direction of a normal to the surface. If the center of the emanating region deviates from the center of the concavity, the center of the emanating region will also deviate from the spherical lens. The light converged by the lens will launch in an oblique direction instead of a vertical direction.
Japanese Patent Laying Open No. 60-161684 (161684/1985) proposed another structure of a lens mount on a semiconductor chip. Instead of boring a circular concavity on a device chip, '684 formed several protrusions on a surface of a chip. The center of the plural protrusions was determined to coincide with the center of the emanating region in the chip. The heights and positions of the protrusions were designed for enabling the spherical lens to come into contact with the edges of all protrusions. The position of the ball lens was completely determined by the protrusions. There was no room for the ball lens (spherical lens) to displace at all. The set of protrusions allowed a unique determination of the position to the lens. The lens was fixed among the protrusions by glue.
In general, the light emitting device has a narrow emanating region and a core of an optical fiber is also narrow in the optoelectronic communication systems. Effective coupling between a fiber and a light emitting device requires a small tolerance less than 2 .mu.m of positioning the spherical lens in the direction parallel with the surface.
However, the light emitting devices on which a lens is directly fixed suffer from the difficulty of positioning. Such a lens-carrying type device has an emanating region on one surface and a lens on the other surface. The first surface with the emanating region is now referred to as a front surface and the second surface having the lens is referred to as a rear surface. The active region (emanating region) has been fabricated on the front surface by the wafer process including the steps of epitaxial growths of layers, photolithography and etching. If the rear surface must be provided with protrusions for retaining a ball lens, the protrusions shall also be produced on the rear surface by another wafer process including the steps of photolithography and dry etching. Thus, the lens-carrying type device requires double wafer processes on the front surface and the rear surface.
Single wafer process allows exact positioning of parts, e.g. an electrode, pn-junctions, active regions, etc. However, the double wafer processes on both surfaces cannot obtain high precision of positioning of the parts which are made on both surfaces. Significant errors surely accompany the double wafer processes treating with both surfaces. The drawback will now be explained more In detail.
A compound semiconductor wafer, e.g. GaAs, InP or GaP wafer is used for producing high emitting devices. The wafer has initially a thickness of 400 .mu.m to 600 .mu.m for facilitating the handling of wafer in the succeeding wafer process. The emitting devices are fabricated by the wafer process comprising the steps of epitaxial growth, impurity diffusion, etching, and forming an electrode on the front surface of the wafer whose rear surface is fixed to a workpiece. The first wafer process makes a lot of equivalent devices on the front surface of the wafer. The emanating region (active layer or pn-junction) has been formed near the front surface.
Then, the wafer is taken off from the workpiece. The wafer is again fixed on a head of a grinding apparatus upside down. The front surface having the emanating region is glued to the supporting head. The rear surface of the wafer (substrate side) is ground till the thickness decreases to about 100 .mu.m to 200 .mu.m. The reduction of the thickness is indispensable to heighten the diffusion of heat which will be yielded by the injection current flowing through the emanating region (pn-junction or active layers). Preferably the wafer should be thinned to a thickness of 100 .mu.m to 200 .mu.m. The grinding treatment eliminates the material by a thickness of about 300 .mu.m to 400 .mu.m from the rear surface of the wafer. The grinding decreases the distance between the rear surface and the emanating region (active layers) to less than 100 .mu.m to 200 .mu.m. The grinding treatment contributes to both enhancing the thermal diffusion and alleviating the absorption of light between the emanating region and the rear surface (emitting surface).
After the grinding treatment, the wafer experiences another wafer process on the rear surface. The front surface is glued to a workpiece for the second wafer process. The second wafer process etches the rear surface for boring concavities or forming protrusions. The positions of the concavities or protrusions should be determined in coincidence with the emanating regions. Therefore, the wafer is treated by two wafer processes on both surfaces. The lens-carrying type devices require double wafer processes applied to the front surface and the rear surface. An emanating region is fabricated per a unit of device on the front surface by the first wafer process. A concavity or a set of protrusions is formed per a unit of device on the rear surface (emitting surface) by the second wafer process. A single wafer process applied on a single surface can achieve high-precision positioning of parts, e.g. diffusion regions, pn-junctions, or an electrode. However, the double wafer processes cannot secure the precise positioning of the parts on different surfaces.
The double wafer processes make a lot of units of light emitting devices on a wafer. The wafer is scribed along cleavage lines and divided into individual device chips. One chip corresponds to one light emitting device. The chip will be mounted on a package with a lens and leads. Electrodes of the device chip will be connected to the leads of the package.
A wafer is 2 inches to 3 inches in diameter. But a chip is a small rectangle with sides of 300 .mu.m to 500 .mu.m. As mentioned so far, in order to fabricate the lens-carrying devices, the emanating regions, or an electrode must be produced on the front surface by the first wafer process, the rear surface must be ground and the concavities or protrusions must be formed on the rear surface by the second wafer process, e.g. etching, evaporation, photolithography, etc.
The tolerance of deviation between the center of the emanating region on the front surface and the center of the concavity or protrusions on the rear surface must be less than 2 .mu.m. However, the wafer has so large diameter and so thin thickness that wafer is likely to warp. The warp of wafer hinders tight contact between the wafer and a mask in photolithography. Air gap between the wafer and mask induces positioning errors of the spots to be exposed by light. Thus, the warp of wafer makes the positioning of parts on both surfaces difficult. The deformation of wafer increases the errors relating to the positioning of parts on two surfaces. Because of the warp of wafer and the double wafer processes, the center of the emanating region on the front surface is apt to deviate from the center of the concavity or protrusions on the rear surface. Thus, the positioning error between the front emanating region and the rear concavity or protrusions often exceeds the tolerance of 2 .mu.m.
The surface-emission type device was provided with a spherical lens by the steps of adjusting the positions on both surfaces, boring a concavity or protrusions and gluing a spherical lens in the concavity or protrusion. Owing to the difficulty of coincidence of positions on two surfaces, the center of a lens does not necessarily coincide with the center of the emanating region within the tolerance. The discrepancy between the lens and the emanating region decreases the coupling efficiency to an optical fiber. Here, the coupling efficiency means the ratio of light beams attaining a fiber core to the total light beams emitted from the emanating region. Therefore, the light emitting devices have to be tested with regard to the coupling efficiency, after the devices have been completely assembled in packages or receptacles with an optical fiber. The coupling efficiency is examined by applying current to the light emitting device and measuring the light intensity at the further end of the optical fiber. Since the lens has been fixed by adhesive, the position of the lens cannot be corrected. Therefore, the samples which reveal a low coupling efficiency less than a predetermined value must be abandoned. Only the samples with a coupling efficiency over the valve pass the test. Thus, prior method of assembling a lens on a device has been annoyed with low yield of products. Low yield has substantially raised the cost of fabrication.
A purpose of this invention is to provide a method of producing lens-bearing light-emitting devices without double wafer processes on both surfaces which tends to invite positioning errors between two surfaces.
Another purpose of this invention is to provide a method of producing lens-bearing devices which can harmonize the center of the emanating region with the center of the lens.
Another purpose of this invention is to provide a method of producing lens-bearing devices which enables to adjust the positions of parts with high precision.
The other purpose of the invention is to provide a method of producing lens-bearing devices with high yield.